Why does a moving bicycle not fall over?

Scientists have shed a new light on a century-long question on why given sufficient forward speed, a bicycle pushed sideways, will not fall over.

The TU Delft team, in collaboration with scientists from Cornell University (USA), centred on the following intriguing question: why is a bicycle self stable, above a certain speed? You add speed to a bike and can then give it a sideways push without it falling over.

Scientists have long been poring over this complicated question, even from as far back as the nineteenth century. Until recently, the consensus within the scientific community was that the stability was very closely related to two factors.

First, the rotating wheels of the bicycle were supposed to provide stability through gyroscopic effects. Secondly, it was thought that the ‘trail’ played an important part. Trail is the distance by which the contact point of the front wheel trails behind the steering axis.

But the new study puts paid to this old notion once and for all.

“We have known for years that the generally accepted explanation for the stability of the bicycle was too simple. Gyroscopic effects and trail do help, but are not essential for stability,” said researcher Arend Schwab of the 3mE faculty at TU Delft.

“In our publication in Science we have now shown not only theoretically but also by means of experiment that our insights are correct,” said Schwab.

Together with doctorate student Jodi Kooijman, he designed and constructed a bicycle with which it could be shown, in an experiment, that both gyroscopic effects and trail are not necessary for a bicycle to remain stable by itself above a certain speed.

This is the so-called Two Mass Skate bicycle. It has small and counter rotating wheels, which mean there is no gyroscopic effect to speak of, and a small negative trail (in other words, where the point of contact of the front wheel is marginally in front of the steering axis). And yet the bicycle remains stable.

Asked if their experimental work has led to the emergence of new theoretical insights, Schwab replied, “We have demonstrated that the mass distribution is also important for stability, especially the location of the centre of mass of the bicycle”s steering mechanism.”

For a bicycle to be stable, the steering mechanism has to be unstable; if the bike falls, the steering should fall even more quickly.